Intravascular stent having high fatigue performance

ABSTRACT

This invention is directed to an expandable stent for implantation in a body lumen, such as an artery, and a method for making it from a single length of tubing. The stent consists of a plurality of radially expandable cylindrical elements generally aligned on a common axis and interconnected by one or more links. A Y-shaped member is comprised of a U-shaped member and a link having a curved portion and a straight portion to improve the flexibility and thereby improve the fatigue performance of the Y-link junction.

BACKGROUND

This invention relates to expandable endoprosthesis devices, generallycalled stents, which are adapted to be implanted into a patient's bodylumen, such as blood vessel, to maintain the patency thereof. Thesedevices are very useful in the treatment of atherosclerotic stenosis inblood vessels.

Various means have been described to deliver and implant stents. Onemethod frequently described for delivering a stent to a desiredintraluminal location includes mounting the expandable stent on anexpandable member, such as a balloon, provided on the distal end of anintravascular catheter, advancing the catheter to the desired locationwithin the patient's body lumen, inflating the balloon on the catheterto expand the stent into a permanent expanded condition and thendeflating the balloon and removing the catheter.

The stent must be able to satisfy a number of mechanical requirements.First, the stent must be capable of withstanding the structural loads,namely radial compressive forces, imposed on the stent as it supportsthe wall of a vessel. Therefore, a stent must possess adequate radialstrength. Radial strength, which is the ability of a stent to resistradial compressive forces, is due to strength and rigidity around acircumferential direction of the stent. Radial strength and rigidity,therefore, may also be described as hoop or circumferential strength andrigidity.

Once expanded, the stent must adequately maintain its size and shapethroughout its service life despite the various forces that may come tobear on it, including the cyclic loading induced by the beating heart.For example, a radially directed force may tend to cause a stent torecoil inward. Generally, it is desirable to minimize recoil.

In addition, the stent must possess sufficient flexibility to allow forcrimping, expansion, and cyclic loading. Longitudinal flexibility isimportant to allow the stent to be maneuvered through a tortuousvascular path and to enable it to conform to a deployment site that maynot be linear or may be subject to flexure. Finally, the stent must bebiocompatible so as not to trigger any adverse vascular responses.

The structure of a stent is typically composed of scaffolding thatincludes a pattern or a network of interconnecting structural elementsoften referred to, in the art, as struts or bar arms. The scaffoldingcan be formed from wires, tubes, or sheets of material rolled into acylindrical shape. The scaffolding is designed so that the stent can beradially compressed (to allow crimping) and radially expanded (to allowdeployment). A conventional stent is allowed to expand and contractthrough the movement of individual structural elements in the patternwith respect to each other.

Additionally, a medicated stent may be fabricated by coating the surfaceof either a metallic or polymeric scaffolding with a polymeric carrierthat includes an active or bioactive agent or drug. Polymericscaffolding may also serve as a carrier of an active agent or drug.

Furthermore, it may be desirable for a stent to be biodegradable. Inmany treatment applications, the presence of a stent in a body may benecessary for a limited period of time until its intended function of,for example, maintaining vascular patency and/or drug delivery isaccomplished. Therefore, stents fabricated from biodegradable,bioabsorbable, and/or bioerodable materials such as bioabsorbablepolymers should be configured to completely erode only after theclinical need for them has ended.

Along with crimping and expansion forces, implanting the stent in thedynamic environment in a curved artery section causes stresses in thestent structure that can result in fatigue failure.

What has been needed and heretofore unavailable is a stent formed from apolymer material and modified in critical structural areas wherecyclical loads are concentrated to reduce fatigue stresses in order toimprove the fatigue performance of the stent.

SUMMARY OF THE INVENTION

The present invention is directed to a polymer expandable stent which isrelatively flexible along its longitudinal axis to facilitate deliverythrough tortuous body lumens, but which has structural features toimprove fatigue performance of the stent.

The stent of the invention generally includes a plurality of radiallyexpandable cylindrical elements or rings which are relativelyindependent in their ability to expand and to flex relatively to oneanother. Typically, the individual radially expandable rings of thestent are dimensioned so as to be longitudinally shorter than their owndiameters. Interconnecting elements or links are used to connectadjacent cylindrical rings and provide increased stability and arepreferably positioned to provide longitudinal flexibility to the stent.The resulting stent structure is a series of radially expandablecylindrical rings which are spaced longitudinally close enough so thatsmall dissections in the wall of a body lumen may be pressed back intoposition, but not so close as to compromise the longitudinal flexibilityof the stent.

Each of the cylindrical rings making up the stent has a proximal end anda distal end and a cylindrical plane defined by a cylindrical outer wallsurface that extends circumferentially between the proximal end and thedistal end of the cylindrical ring. Generally the cylindrical rings havea serpentine or undulating shape which includes at least one U-shapedelement, and typically each ring has more than one U-shaped element. Thecylindrical rings are interconnected by at least one link which attachesone cylindrical ring to an adjacent cylindrical ring. The combination ofthe rings and links forms the stent pattern and allows the stent to behighly flexible along its longitudinal axis.

Not only do the links that interconnect the cylindrical rings provideflexibility to the stent, but the positioning of the links also enhancesthe flexibility by allowing uniform flexibility when the stent is bentin any direction along its longitudinal axis. Uniform flexibility alongthe stent derives in part from the links in one ring beingcircumferentially offset from the links in an adjacent ring. Further,the cylindrical rings are configured to provide flexibility to the stentin that portions of the rings can flex or bend as the stent is deliveredthrough a tortuous vessel.

The cylindrical rings typically are formed of a plurality of peaks andvalleys, where the valleys of one cylindrical ring are circumferentiallyaligned with the valleys of an adjacent cylindrical ring, which is knownin the art as being in-phase. In this configuration, at least one linkattaches each cylindrical ring to an adjacent cylindrical ring so thatat least a portion of the link is positioned within one of the valleys,and it attaches the valley to an adjacent valley.

While the cylindrical rings and links generally are not separatestructures, they have been conveniently referred to as rings and linksfor ease of identification. Further, the cylindrical rings can bethought of as comprising a series of U, W and Y-shaped structures in arepeating pattern. Again, while the cylindrical rings are not divided upor segmented into U's, W's and Y's, the pattern of the cylindrical ringsresembles such configurations. The U's, W's and Y's promote flexibilityin the stent primarily by flexing as the stent is delivered through atortuous vessel.

The radial expansion of the expandable rings deforms the undulatingpattern thereof similar to changes in a waveform which result indecreasing the waveform's amplitude. Preferably, the undulating patternsof the individual cylindrical structures are in phase with each other inorder to prevent contraction of the stent along its length when it isexpanded. The cylindrical structures of the stent are plasticallydeformed when expanded so that the stent will remain in the expandedcondition, and therefore they must be sufficiently rigid when expandedto prevent the collapse thereof in use.

The links may be formed in a unitary structure with the expandablecylindrical rings from the same intermediate product, such as a tubularelement. Preferably, all of the links of a stent are joined at thevalleys of the undulating structure of the cylindrical rings which formthe stent. In this manner, there is no shortening of the stent uponexpansion.

The number and location of the links interconnecting adjacentcylindrical rings can be varied in order to develop the desiredlongitudinal flexibility in the stent structure both in the unexpandedas well as the expanded condition. These properties are important tominimize alteration to the natural physiology of the body lumen intowhich the stent is implanted and to maintain the compliance of the bodylumen which is internally supported by the stent. Generally, the greaterthe longitudinal flexibility of the stent, the easier and the moresafely it can be delivered to the implantation site.

In one embodiment, the Y-shaped members are formed by the linksattaching to the curved portion of a valley, which then resembles theY-shaped member. In this embodiment, each link has a straight sectionand a curved section. The curved section extends in a circumferentialdirection. The curved section has a first curved length and a secondcurved length, the first curved length being greater than the secondcurved length. Further, the straight section has a length, and thecurved section has a length with the length of the curved section beinggreater than the length of the straight section. The curved section inthe links greatly improves the fatigue resistance of the links,especially at the interface of the curved section and the valley of acylindrical ring.

Other features and advantages of the present invention will become moreapparent from the following detailed description of the invention, whentaken in conjunction with the accompanying exemplary drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational view, partially in section, of a prior artstent with curved links.

FIG. 2 is an elevational view, partially in section, similar to thatshown in FIG. 1 wherein the prior art stent is expanded within anartery.

FIG. 3 is an elevational view, partially in section, showing theexpanded prior art stent implanted in the artery after withdrawal of thedelivery catheter.

FIG. 4 is a plan view of a flattened prior art stent which illustratesthe curved links of the stent shown in FIG. 5.

FIG. 5 is a plan view of the prior art stent of FIG. 4 in an unexpandedstate.

FIG. 6 is a plan view of a flattened section of one embodiment of thestent depicting curved links.

FIG. 7 is an enlarged partial view of a Y-shaped member depictingfeatures of the invention including a link having a straight portion anda curved portion.

FIG. 8A is an enlarged partial view of a Y-shaped member depicting thelengths of the bends and the straight sections of the link.

FIG. 8B is an enlarged partial view of a Y-shaped member depicting theradii of the curved portions of the link.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning to the drawings, FIG. 1 depicts a prior art stent 10 mounted ona conventional catheter assembly 12 which is used to deliver the stentand implant it in a body lumen, such as a coronary artery, peripheralartery, or other vessel or lumen within the body. The catheter assemblyincludes a catheter shaft 13 which has a proximal end 14 and a distalend 16. The catheter assembly is configured to advance through thepatient's vascular system by advancing over a guide wire 18 by any ofthe well-known methods of an over the wire system (not shown) or awell-known rapid exchange catheter system, such as the one shown in FIG.1.

Catheter assembly 12 as depicted in FIG. 1 is of the well-known rapidexchange type which includes an RX port 20 where the guide wire 18 exitsthe catheter. The distal end of the guide wire 18 exits the catheterdistal end 16 so that the catheter advances along the guide wire on asection of the catheter between the RX port 20 and the catheter distalend 16. As is known in the art, the guide wire lumen which receives theguide wire is sized for receiving various diameter guide wires to suit aparticular application. The stent is mounted on the expandable member 22(balloon) and is crimped tightly thereon so that the stent andexpandable member present a low profile diameter for delivery throughthe arteries.

As shown in FIG. 1, a partial cross-section of an artery 24 is shownwith a small amount of plaque that has been previously treated by anangioplasty or other repair procedure. Stent 10 is used to repair adiseased or damaged arterial wall which may include the plaque 26 asshown in FIG. 1, or a dissection, or a flap which are sometimes found incoronary arteries, peripheral arteries and other vessels.

In a typical procedure to implant prior art stent 10, the guide wire 18is advanced through the patient's vascular system by well-known methodsso that the distal end of the guide wire is advanced past the plaque ordiseased area 26. Prior to implanting the stent, the cardiologist maywish to perform an angioplasty procedure or other procedure (i.e.,atherectomy) in order to open the vessel and remodel the diseased area.Thereafter, the stent delivery catheter assembly 12 is advanced over theguide wire 18 so that the stent is positioned in the target area. Theexpandable member or balloon 22 is inflated by well-known means so thatit expands radially outwardly and in turn expands the stent radiallyoutwardly until the stent is apposed to the vessel wall. The expandablemember is then deflated, and the catheter is withdrawn from thepatient's vascular system. The guide wire typically is left in the lumenfor post-dilatation procedures, if any, and subsequently is withdrawnfrom the patient's vascular system. As depicted in FIG. 2, the balloonis fully inflated with the prior art stent expanded and pressed againstthe vessel wall, and in FIG. 3, the implanted stent remains in thevessel after the balloon has been deflated, and the catheter assemblyand guide wire have been withdrawn from the patient.

The prior art stent 10 serves to hold open the artery after the catheteris withdrawn, as illustrated by FIG. 3. Due to the formation of thestent from a thin walled tubular member, the undulating components ofthe stent are relatively flat in transverse cross-section so that whenthe stent is expanded, they are pressed into the wall of the artery and,as a result, do not interfere with the blood flow through the artery.The stent is pressed into the wall of the artery and will eventually becovered with endothelial cell growth which further minimizes blood flowinterference. Plastic deformation of the undulating rings of the stentduring expansion provides good tacking characteristics to prevent stentmovement within the artery. Furthermore, the closely spaced cylindricalelements at regular intervals provide uniform support for the wall ofthe artery and consequently are well adapted to tack up and hold inplace any small flaps or dissections in the wall of the artery.

One of the problems associated with some prior art stents such as theone shown in FIG. 4 is the ability to more tightly crimp or compress thestent 10 onto the balloon portion of the catheter. For example, theundulating portion 27 of the links 28 of the prior art stent in FIG. 4is positioned between two struts 29A and 29B so that as the stent canonly be tightly crimped or compressed onto the balloon portion of thecatheter before contact between the undulating portion of the link andthe struts is made. Preferably, the undulating portion of the link andthe adjacent struts should not overlap, therefore the undulating portionof the link limits the amount of the crimping or compression of eachcylindrical ring onto the balloon portion of the catheter. The presentinvention solves this problem and allows for a tightly compressed orcrimped stent onto the catheter. Further, since the stent is formed froma polymer material, there may be some deformation of the curved linkduring crimping and/or expansion.

The underlying structure or substrate of a stent can be completely or atleast in part made from a biodegradable polymer or combination ofbiodegradable polymers, a biostable polymer or combination of biostablepolymers, or a combination of biodegradable and biostable polymers.Additionally, a polymer-based coating for the surface of a device can bea biodegradable polymer or combination of biodegradable polymers, abiostable polymer or combination of biostable polymers, or a combinationof biodegradable and biostable polymers.

A stent may be fabricated from a polymeric tube or a polymeric sheet byrolling and bonding the sheet edges to form a tube. A stent pattern maybe formed on a polymeric tube by laser cutting a pattern on the tube.Representative examples of lasers that may be used include, but are notlimited to, excimer, carbon dioxide, and YAG. In other embodiments,chemical etching may be used to form a stent pattern on a tube.

The pattern of a stent can vary throughout its structure to allow radialexpansion and compression and longitudinal flexure. A pattern mayinclude portions of struts that are straight or relatively straight, orsome patterns may include bending elements. The bending elements cancomprise of bending elements such as W-shaped bending elements andY-shaped bending elements. Bending elements that have a U-shape are freebending elements. The free bending elements are not directly connectedto any adjacent ring, whereas each W-shaped bending element is directlyconnected by one of the links at a concave portion of the apex of theW-shaped bending element to a convex portion of an apex on an adjacentring located in a direction of a first end of a scaffold, and eachY-shaped bending element is directly connected by one of the links at aconvex portion of the apex of the Y-shaped bending element to a concaveportion of an apex on an adjacent ring located in a direction of asecond end of the scaffold.

Bending elements bend inward when a stent is crimped to allow for radialcompression. Bending elements also bend outward when a stent is expandedto allow for radial expansion. After deployment, a stent is under staticand cyclic compressive loads from the vessel wall. Thus, bendingelements are subjected to deformation during use. “Use” includes, but isnot limited to, manufacturing, assembling (e.g., crimping stent on acatheter), delivery of stent into and through a bodily lumen to atreatment site, deployment of stent at a treatment site, and treatmentafter deployment.

As indicated above, a stent is required to have certain mechanicalrequirements. A stent must have sufficient radial strength to withstandstructural loads, namely radial compressive forces, imposed on the stentas it supports the wall of a vessel. In addition, the stent must possesssufficient flexibility to allow for crimping, expansion, and cyclicloading. Also, a sufficiently low profile, that includes crimpeddiameter and strut thickness, is important. As the profile of a stentdecreases due to the stent being crimped more tightly on the balloonand/or thinner strut thickness, as the strut thickness decreases, itsdelivery to the treatment site is easier, and the disruption to theblood flow caused by the strut thickness decreases.

Polymers tend to have a number of shortcomings for use as materials forstents. One such shortcoming is that many biodegradable polymers have arelatively low modulus, and thus relatively low radial strength.Compared to metals, the strength to weight ratio of polymers is smallerthan that of metals. A polymeric stent with inadequate radial strengthcan result in mechanical failure or recoil inward after implantationinto a vessel. To compensate for the relatively low modulus, a polymericstent requires significantly thicker struts than a metallic stent, whichresults in an undesirably large profile.

Another shortcoming of polymers is that many polymers, such asbiodegradable polymers, tend to be brittle under physiologicalconditions or conditions within a human body. Specifically, suchpolymers can have a Tg, which is defined below, above human bodytemperature which is approximately 37° C. These polymer systems exhibita brittle fracture mechanism in which there is little or no plasticdeformation prior to failure. As a result, a stent fabricated from suchpolymers can have insufficient toughness for the range of use of astent. In particular, it is important for a stent to be resistant tofracture throughout the range of its use, i.e., crimping, delivery,deployment, and post-deployment during a desired treatment period.

The “glass transition temperature,” Tg, is the temperature at which theamorphous domains of a polymer change from a brittle vitreous state to asolid deformable or ductile state at atmospheric pressure. In otherwords, the Tg corresponds to the temperature where the onset ofsegmental motion in the chains of the polymer occurs. When an amorphousor semicrystalline polymer is exposed to an increasing temperature, thecoefficient of expansion and the heat capacity of the polymer bothincrease as the temperature is raised, indicating increased molecularmotion. As the temperature is raised, the actual molecular volume in thesample remains constant, and so a higher coefficient of expansion pointsto an increase in free volume associated with the system and thereforeincreases freedom for the molecules to move. The increasing heatcapacity corresponds to an increase in heat dissipation throughmovement. Tg of a given polymer can be dependent on the heating rate andcan be influenced by the thermal history of the polymer. Furthermore,the chemical structure of the polymer heavily influences the glasstransition by affecting mobility.

Other potential problems with polymeric stents include creep, stressrelaxation, and physical aging. Creep refers to the gradual deformationthat occurs in a polymeric construct subjected to an applied load. It isbelieved that the delayed response of polymer chains to stress duringdeformation causes creep behavior. Creep occurs even when the appliedload is constant. Creep can cause an expanded stent to retract radiallyinward, reducing the effectiveness of a stent in maintaining desiredvascular patency. The rate at which polymers creep depends not only onthe load, but also on temperature. In general, a loaded construct creepsfaster at higher temperatures.

Stress relaxation is also a consequence of delayed molecular motions asin creep. Contrary to creep, however, which is experienced when the loadis constant, stress relaxation occurs when deformation (or strain) isconstant and is manifested by a reduction in the force (stress) requiredto maintain a constant deformation

Physical aging, as used herein, refers to densification in the amorphousregions of a semi-crystalline polymer. Physical aging ofsemi-crystalline polymers that have glass transition temperatures (Tg)above their normal storage temperature, which, for the purposes of thisinvention is room temperature, i.e., from about 15° C. to about 35° C.,occurs primarily through the phenomenon known as densification.Densification occurs when polymer chains rearrange in order to move froma non-equilibrium state to an equilibrium state. The reordering ofpolymer chains tends to increase the modulus of the polymer resulting ina brittle or more brittle polymer.

Thus, physical aging results in an increase in brittleness of a polymerwhich can result in cracking of struts upon crimping and deployment.Since physical aging results from densification of amorphous regions ofa polymer, an increase in crystallinity can reduce or inhibit physicalaging.

However, it is well known by those skilled in the art that themechanical properties of a polymer can be modified through variousprocessing techniques, such as, by applying stress to a polymer. JamesL. White and Joseph E. Spruiell, Polymer and Engineering Science, 1981,Vol. 21, No. 13. The application of stress can induce molecularorientation along the direction of stress which can modify mechanicalproperties along the direction of applied stress. For example, strengthand modulus are some of the important properties that depend uponorientation of polymer chains in a polymer. Molecular orientation refersto the relative orientation of polymer chains along a longitudinal orcovalent axis of the polymer chains.

A polymer may be completely amorphous, partially crystalline, or almostcompletely crystalline. A partially crystalline polymer includescrystalline regions separated by amorphous regions. The crystallineregions do not necessarily have the same or similar orientation ofpolymer chains. However, a high degree of orientation of crystallitesmay be induced by applying stress to a semi-crystalline polymer. Thestress may also induce orientation in the amorphous regions. An orientedamorphous region also tends to have high strength and high modulus alongan axis of alignment of polymer chains. Additionally, for some polymersunder some conditions, induced alignment in an amorphous polymer may beaccompanied by crystallization of the amorphous polymer into an orderedstructure. This is known as stress induced crystallization.

As indicated above, due to the magnitude and directions of stressesimposed on a stent during use, it is important for the mechanicalstability of the stent to have suitable mechanical properties, such asstrength and modulus, in the axial and circumferential directions.Therefore, it can be advantageous to modify the mechanical properties ofa tube, to be used in the fabrication of a stent, by induced orientationfrom applied stress in the axial direction, circumferential direction,or both. Since highly oriented regions in polymers tend to be associatedwith higher strength and modulus, it may be desirable to incorporateprocesses that induce alignment of polymer chains along one or morepreferred axes or directions into fabrication of stents.

In keeping with the invention and referring to FIG. 6, stent 40 is shownin a flattened condition so that the pattern can be clearly viewed, eventhough the stent is in a cylindrical form in use. The stent is typicallyformed from a tubular member, however, it can be formed from a flatsheet such as shown in FIG. 6 and rolled into a cylindricalconfiguration. The stent 40 is intended to be formed of a polymermaterial. In one embodiment, the polymer material is processed so thatthe crystalline direction is primarily in the circumferential direction,thereby providing the necessary radial strength. This in partcontributes to the Y-link junction being a critical location forfatigue. The design improvements disclosed herein improve the fatigueperformance of the critical Y-link junction.

As shown in FIG. 6, stent 40 is made up of a plurality of cylindricalrings 42 which extend circumferentially around the stent when it is in atubular form. Each cylindrical ring 42 has a cylindrical ring distal end44 and a cylindrical ring proximal end 46. Typically, since the stent islaser cut from a tube, there are no discreet parts such as the describedcylindrical rings and links. However, it is beneficial foridentification and reference to various parts to refer to thecylindrical rings and links and other parts of the stent as follows.Each cylindrical ring 42 is comprised of U-shaped members 48, Y-shapedmembers 50, and W-shaped members 52. Undulating links 54 attach adjacentrings and form part of the Y-shaped members and the W-shaped members.

Each cylindrical ring 42 defines a cylindrical plane which is a planedefined by the distal and proximal ends 44, 46 of the ring and thecircumferential extent as the cylindrical ring travels around thecylinder. Each cylindrical ring includes cylindrical outer wall surfacewhich defines the outermost surface of the stent, and cylindrical innerwall surface which defines the innermost surface of the stent. Thecylindrical plane follows the cylindrical outer wall surface.

The undulating links 54 are positioned within the cylindrical plane. InFIG. 6, the undulating portion of the link is positioned betweenadjacent rings. The undulating links connect one cylindrical ring to anadjacent cylindrical ring and contribute to the overall longitudinalflexibility of the stent due to their unique construction. Theflexibility of the undulating links derives in part from a curvedportion 56 connected to a straight portion 58, wherein the straightportion is substantially parallel to the longitudinal axis of the stent.Thus, as the stent is being delivered through a tortuous vessel, such asa coronary artery, the curved portions 56 of the undulating links 54will permit the stent to flex in the longitudinal direction whichsubstantially enhances delivery of the stent to the target site. Incontrast, a straight portion 58 is parallel to the stent axis and has nocurved portion, therefore it is not as flexible as the curved portionand does not add to the flexibility of the stent. An important featureof the present invention is to increase the structural fatigueperformance of the links in view of the arterial curvature when thestent is implanted in a coronary artery.

In one embodiment, as shown in FIG. 7, the stent section 78 has aY-shaped member 80 formed by a link 82 attaching to the curved portion84 of a valley 86, which then resembles the Y-shaped member. In thisembodiment, each link 82 has straight section 88 and a curved section90. The curved section 90 extends in a circumferential direction. Thecurved section 90 has a first curved length 92 and a second curvedlength 94, the first curved length being greater than the second curvedlength. Furthermore, the straight section 88 has a length 96 which issignificantly less than both curved lengths 92 and 94 of the curvedsection 90. The curved section 90 in each link 82 greatly improves thefatigue resistance of the links, especially at the interface of thecurved section 90 and the valley 86 of a cylindrical ring.

In one embodiment, as shown in FIG. 7, the Y-shaped members 80 areformed by links 82 attaching to the curved portion 84 of a valley 86,which then resembles the Y-shaped member. Each link has a straightsection 88 and a curved section 90. The links 82 have a longitudinalaxis LA bisecting the width of the links 82. In this embodiment, thecurved section 90 has a width equal to the width of the links. Thecurved section has a first curved length 92 and a second curved length94. The second curved length 94 has an arc line 102 that is tangent tothe longitudinal axis LA of the links. In this embodiment, the curvedsection 90 is formed by bending the links 82 a distance equal to onehalf of the width of the links, however, the curved section 90 can havedifferent curved lengths and still be within the invention. Of course,the links 82 are not literally bent to form the curved section 90 sincethe stent of the invention is formed by laser cutting a tubular memberusing a well-known process. Thus, reference to bending the links is forillustration purposes only. Importantly, the first curved length 92 andthe second curved length 94 are comprised of curves having differentradii so that the curved section provides improved link structuralfatigue performance. In this embodiment, the first curved length is inthe range from 0.035 inch (0.89 mm) to 0.028 inch (0.71 mm) and thesecond curved length is in the range from 0.027 inch (0.69 mm) to 0.018inch (0.46 mm). In one preferred embodiment, the first curved length is0.027 inch (0.68 mm) and the second curved length is 0.024 inch (0.60mm). For one embodiment, in fatigue testing comparing a straight linkversus the present invention with a curved section 90 in the links 82,there was a 23% increase in the links' structural fatigue performance.

In an embodiment shown in FIG. 8A, the length dimensions for thestructural bends in the links are disclosed. The length along thelongitudinal stent axis of the link outer radius 110 is 0.027 inch (0.68mm); the length of the link outer straight portion 112 is 0.005 inch(0.13 mm); the length of the link inner radius 114 is 0.024 inch (0.60mm); the length of the link inner straight portion 116 is 0.007 inch(0.17 mm); and the distance moving circumferentially from the innerstraight portion 116 to the inner peak 118 of the inner radius is 0.003inch (0.08 mm). Further, the distance moving circumferentially from theinner peak 118 to the outer straight portion 112 is also 0.003 inch(0.08 mm). These lengths, coupled with the various radii disclosedherein, provide for undulating links having increased fatigueperformance.

Referring to FIG. 8B, the elements A-H represent the various radii ofthe undulating link 110 so that the curved section represented byelements A-H is formed by bending a straight link a distance equal toone half of the width of the link. Of course, the links are notliterally bent since they are laser cut from a tube, but the result isthe inner curve section (elements F and G) being essentially tangent toa line that is the longitudinal centerline of the link if it was astraight link. The strategic placement of the various radii (elementsA-H) provides for undulating links 110 having increased fatigueperformance. Table 1, immediately below, discloses the various radii forelements A-H.

Radius Length Distance Element inch/mm inch/mm inch/mm A .012/0.30.006/0.14 .006/0.14 B .016/0.41 .007/0.19 .007/0.19 C .023/0.59.008/0.20 .008/0.20 D .004/0.10 .006/0.14 .005/0.12 E .012/0.30.006/0.16 .006/0.15 F .010/0.25 .006/0.14 .005/0.13 G .017/0.44.012/0.30 .011/0.28 H .006/0.16 .004/0.10 .002/0.06

While the invention has been illustrated and described herein in termsof its use as an intravascular stent for treating coronary arteries, itwill be apparent to those skilled in the art that the stent can be usedin other instances such as in treating peripheral vessels. Othermodifications and improvements may be made without departing from thescope of the invention.

Other modifications and improvements can be made to the inventionwithout departing from the scope thereof.

We claim:
 1. A longitudinally flexible stent for implanting in a bodylumen, comprising: a plurality of cylindrical rings including a firstcylindrical ring, a second cylindrical ring, a third cylindrical ring,up to an eighteenth cylindrical ring, the cylindrical rings beinggenerally independently expandable in the radial direction and generallyaligned on a common longitudinal axis; each of the cylindrical ringshaving an undulating pattern of peaks and valleys, the undulatingpattern of each of the cylindrical rings being in phase with theundulating pattern of each of the adjacent cylindrical rings; each ofthe cylindrical rings being interconnected by links to one of theadjacent cylindrical rings so that the cylindrical rings form alongitudinally flexible stent; and each of the links having a straightsection and a curved section, the curved section having a first curvedlength and a second curved length and wherein the first curved length isin the range from 0.035 inch (0.89 mm) to 0.028 inch (0.71 mm), and thesecond curved length is in the range from 0.027 inch (0.69 mm) to 0.018inch (0.46 mm).
 2. The stent of claim 1, wherein the straight section ofeach link has a length, and the curved section has a length, the lengthof the curved section being greater than the length of the straightsection.
 3. The stent of claim 2, wherein the first curved length ofeach link is greater than the second curved length.
 4. The stent ofclaim 2, wherein the links have a width that is uniform along thestraight section and the curved section.
 5. The stent of claim 3,wherein the first curved length is 0.027 inch (0.68 mm) and the secondcurved length is 0.024 inch (0.60 mm).
 6. The stent of claim 1, whereinthe curved section of each link is in a circumferential direction. 7.The stent of claim 1, wherein the distance between adjacent cylindricalrings is more than a width of either a single peak or a single valley.8. The stent of claim 1, wherein the second curved length of each linkforms an arc line that is tangent to a longitudinal axis of each link.